Nitroxyl anion (NO ؊ ) is the one-electron reduction product of nitric oxide (NO ⅐ ) and is enzymatically generated by NO synthase in vitro. The physiologic activity and mechanism of action of NO ؊ in vivo remains unknown. The NO ؊ generator Angeli's salt (AS, Na2N2O3) was administered to conscious chronically instrumented dogs, and pressure-dimension analysis was used to discriminate contractile from peripheral vascular responses. AS rapidly enhanced left ventricular contractility and concomitantly lowered cardiac preload volume and diastolic pressure (venodilation) without a change in arterial resistance. There were no associated changes in arterial or venous plasma cGMP. The inotropic response was similar despite reflex blockade with hexamethonium or volume reexpansion, indicating its independence from baroreflex stimulation. However, reflex activation did play a major role in the selective venodilation observed under basal conditions. These data contrasted with the pure NO donor diethylamine͞NO, which induced a negligible inotropic response and a more balanced veno͞arterial dilation. AS-induced positive inotropy, but not systemic vasodilatation, was highly redox-sensitive, being virtually inhibited by coinfusion of N-acetyl-L-cysteine. Cardiac inotropic signaling by NO ؊ was mediated by calcitonin gene-related peptide (CGRP), as treatment with the selective CGRP-receptor antagonist CGRP-(8 -37) prevented this effect but not systemic vasodilation. Thus, NO ؊ is a redox-sensitive positive inotrope with selective venodilator action, whose cardiac effects are mediated by CGRP-receptor stimulation. This fact is evidence linking NO ؊ to redox-sensitive cardiac contractile modulation by nonadrenergic͞noncholinergic peptide signaling. Given its cardiac and vascular properties, NO ؊ may prove useful for the treatment of cardiovascular diseases characterized by cardiac depression and elevated venous filling pressures.N itric oxide (NO ⅐ )-related species play a crucial role in diverse physiological processes, including blood pressure regulation, neurotransmission, and cytostatic͞cytotoxic signaling (1). Whereas some NO ⅐ -mediated cardiovascular effects are firmly established, its control over myocardial contractility remains controversial in that positive, negative, or neutral effects can be observed. The net result varies with the tissue preparation, NO ⅐ concentration and donor, and myocardial redox state (2, 3). These factors can critically influence the particular NO ⅐ species generated and, thereby, the net contractile response.Among the NO ⅐ -related species is nitroxyl anion (NO Ϫ ), the one-electron reduction product of NO ⅐ that is formed by NO ⅐ synthase in vitro by direct enzyme action or metabolism of the decoupled NO ⅐ synthase product N G -hydroxy-L-arginine (4-8). At high concentrations of 0.1-5 mM, NO Ϫ seems more cytotoxic than NO ⅐ in vitro, causing DNA strand breaks and base oxidation (9, 10). Like NO ⅐ , NO Ϫ induces vasodilation in vivo and in vitro in association with the formation of iron-nitro...

Abstract-␤-Adrenergic agonists stimulate cardiac contractility and simultaneously blunt this response by coactivating NO synthase (NOS3) to enhance cGMP synthesis and activate protein kinase G (PKG-1). cGMP is also catabolically regulated by phosphodiesterase 5A (PDE5A). PDE5A inhibition by sildenafil (Viagra) increases cGMP and is used widely to treat erectile dysfunction; however, its role in the heart and its interaction with ␤-adrenergic and NOS3/cGMP stimulation is largely unknown. In nontransgenic (control) murine in vivo hearts and isolated myocytes, PDE5A inhibition (sildenafil) minimally altered rest function. However, when the hearts or isolated myocytes were stimulated with isoproterenol, PDE5A inhibition was associated with a suppression of contractility that was coupled to elevated cGMP and increased PKG-1 activity. In contrast, NOS3-null hearts or controls with NOS inhibited by N G -nitro-Larginine methyl ester, or soluble guanylate cyclase (sGC) inhibited by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one, showed no effect of PDE5A inhibition on ␤-stimulated contractility or PKG-1 activation. This lack of response was not attributable to altered PDE5A gene or protein expression or in vitro PDE5A activity, but rather to an absence of sGC-generated cGMP specifically targeted to PDE5A catabolism and to a loss of PDE5A localization to z-bands. Re-expression of active NOS3 in NOS3-null hearts by adenoviral gene transfer restored PDE5A z-band localization and the antiadrenergic efficacy of PDE5A inhibition. These data support a novel regulatory role of PDE5A in hearts under adrenergic stimulation and highlight specific coupling of PDE5A catabolic regulation with NOS3-derived cGMP attributable to protein subcellular localization and targeted synthetic/catabolic coupling. Key Words: PDE5 Ⅲ phosphodiesterase Ⅲ sildenafil Ⅲ nitric oxide synthase Ⅲ contractility Ⅲ z-band B eta-adrenergic regulation of cardiac contraction is coupled to elevations in adenosine (cAMP) and guanosine (cGMP) cyclic nucleotides. Increased cAMP enhances contractility 1,2 by activating protein kinase A (PKA), whereas concomitant stimulation of cGMP opposes this in part by activating protein kinase G (PKG-1). 3,4 The latter response is thought to be attributable to stimulation of soluble guanylate cyclase (sGC) by NO. 3-9 Cyclic GMP is also synthesized by receptor GC (rGC) coupled to natriuretic peptide stimulation, and both sources can modulate cardiac function and structure, particularly in hearts stimulated by neurohormones or mechanical stress. 3-5,10 -13 cGMP is also regulated by catabolic phosphodiesterases such as phosphodiesterase 5A (PDE5A), and PDE5A inhibition by sildenafil (Viagra; SIL) and similar compounds augments cGMP in vascular tissue and is the primary therapy for erectile dysfunction. 14,15 However, the role for PDE5A in regulating cardiac function has remained unclear. 16 -18 Such clarification has become increasingly important because PDE5A inhibitors are poised to become chronic treatments for diseases such as pul...